The removal of sulfur components, particularly hydrogen sulfide, from gaseous streams such as the waste gases liberated in the course of various chemical and industrial processes, for example, in the pulping of wood, in the production of natural gas and crude oil and in petroleum refining, has become increasingly important in combating atmospheric pollution. Gases containing sulfurs, particularly hydrogen sulfide, not only have an offensive odor, but such gases may also cause damage to vegetation, painted surfaces, and wild life as well as constitute a significant health hazard to humans. Government wide regulations have increasingly imposed continuously lower tolerances on the sulfur content of gases which can be vented to the atmosphere, and it is now imperative in many localities to remove virtually all the sulfurs, particularly hydrogen sulfide, under the penalty of an absolute ban on continuing operation of a plant or the like which produces the sulfur-containing gaseous stream.
In the majority of cases, sulfurs in a gaseous stream are substantially converted to hydrogen sulfide by well known means prior to subjecting the gaseous stream to treatment to remove the sulfur components therein. One well known process for the removal of hydrogen sulfide from gaseous streams is known as the Claus process. According to the Claus process, elemental sulfur is produced from hydrogen sulfide by (a) partially oxidizing the hydrogen sulfide in the gas stream to sulfur dioxide by means of atmospheric oxygen and (b) subsequently reacting the sulfur dioxide formed with the remaining portion of the hydrogen sulfide in the presence of a catalyst. The Claus system comprises a combustion chamber in which, at temperatures from 950.degree. to 1350.degree. C., about 50 to 70% of the sulfur contained in the feed gas is converted into elemental sulfur. The sulfur is condensed out by cooling the reaction gas to a temperature below the dew point of sulfur. After the separation of the sulfur, the remaining gases are heated to a temperature above the dew point of sulfur and the treatment is repeated to further remove sulfur components. Normally, the gas passes through not less than two such Claus catalyst stages between which the reaction gas is cooled to a temperature below the sulfur dew point, the condensed sulfur is removed and the remaining gas is reheated before entering the next Claus catalyst bed.
The different stages of the process may be represented by the following equations: ##EQU1##
The overall reaction may be expressed by the following equation: ##EQU2##
Due to the given thermodynamic equilibrium and process conditions, the exhaust gas of the last catalytic process stage still contains small amounts of hydrogen sulfide, sulfur dioxide, carbon oxysulfide, carbon disulfide, and elemental sulfur in the form of vapor or mist. Such gas is subjected to an after combustion wherein the sulfur components are converted into sulfur dioxide which is then released into the atmosphere. Depending on the way in which the process is conducted, the sulfur emitted into the atmosphere with the exhaust gas in the form of sulfur dioxide still amounts to about 2 to 6% of the sulfur contained in the feed gas in the form of hydrogen sulfide. In view of the air pollution and the loss of sulfur involved, further purification is imperative.
To reduce the emission of sulfur compounds from Claus systems, a number of special processes for after treatment have been developed and tested. Such after treatment is carried out either directly after the last catalytic process stage or before the post combustion, depending on the type of process. These additional after treatment installations are complicated and expensive in regard to apparatus and process technology due to the diversity of the sulfur compounds occuring in the Claus exhaust gas.
One group of processes, applied before the post-combustion, utilizes catalytic reaction, to achieve as complete a reaction process as possible between hydrogen sulfide and sulfur dioxide. With these processes, the reaction temperature is lowered to below the condensation point of sulfur, whereby the reaction equilibrium corresponding to equation II is shifted in the direction favoring the formation of sulfur. In this group, one distinguishes between dry processes using alternating reactors in which the catalyst is intermittently charged with sulfur and discharged, and processes wherein hydrogen sulfide and sulfur dioxide react in high-oiling catalyst-containing liquid, forming elemental sulfur, where the latter is drawn off continuously as a liquid product. These processes have disadvantages in that any deviation from the optimum hydrogen sulfide/sulfur dioxide ratio in the Claus exhaust gas results in reduced sulfur yield and that no appreciable conversion of sulfur compounds, such as carbon oxysulfide and carbon disulfide takes place.
With another group of processes, a gas containing hydrogen or carbon monoxide is utilized in the presence of water for the hydrogenation of the sulfur components into hydrogen sulfide. The hydrogen sulfide is further processed by either (a) absorptive scrubbing processes with regeneration wherein removed hydrogen sulfide is returned into the Claus system; or (b) oxidative scrubbing processes wherein removed hydrogen sulfide in the solution is directly converted into elemental sulfur. These processes do not require a stoichiometric hydrogen sulfide/sulfur dioxide ratio in the Claus exhaust. However, these processes require high expenditures for elaborate apparatus and have high energy requirements. The return of washed out hydrogen sulfide curtails the Claus capacity. Furthermore, some of the processes in this group produce waste water containing harmful constituents.
It is also known to increase the equilibrium conversion of the Claus reaction (equation II) by condensing out part of the water contained in the gas to be purified. The gas is subsequently heated to the temperature required for a Claus reactor and caused to react over a Claus catalyst to form elemental sulfur. A disadvantage in this connection is that this process produces waste water that is highly corrosive due to the formation of thiosulfuric acid, polythionic acids and sulfurous acid and any processing of such waste water requires high expenditure. Problems are also caused by the unavoidable formation of deposits of elemental sulfur during water condensation. There is no conversion of carbon oxysulfide and carbon disulfide with this process.
It is likewise known to proceed, after the mentioned conversion of all sulfur compounds into hydrogen sulfide, by oxidizing part of said hydrogen sulfide with air into sulfur dioxide or converting part of the sulfur produced into sulfur dioxide and further catalytically converting the remaining hydrogen sulfide with sulfur dioxide, at 125.degree.-150.degree. C. in fixed-Beol reactors, into sulfur and regenerating the loaded catalyst by passing through it hot oxygen-free gases containing hydrogen sulfide. Thereby, it is possible to avoid the disadvantages of the processes described in connection with the first group, such as dependence on hydrogen sulfide/sulfur dioxide ratio and carbon oxysulfide/carbon disulfide content in the Claus exhaust gas. Disadvantages include the increased apparatus expenditure required by the addition of the hydrogenation/hydrolysis stage and the higher hydrogen sulfide/sulfur dioxide input concentration for the low-temperature reactor caused by the admixture of the separately produced flow of sulfur dioxide.
Moreover, there are known processes for direct catalytic oxidation of hydrogen sulfide in gas mixtures with air or oxygen into elemental sulfur. These processes have several disadvantages. Partly, they are not sufficiently effective in the thermodynamically advantageous temperature range or the proposed catalysts quickly lose their activity. The conversion efficiency obtainable is poor, particularly with low sulfur concentrations on account of the unfavorable reaction kinetics at the low temperatures. Some of these processes lack sufficient selectivity for hydrogen sulfide oxidation so that, partly, other oxidizable compounds, such as hydrogen, carbon monoxide and hydrocarbons are oxidized as well. To avoid this, the hydrogen sulfide oxidation is also carried out at temperatures below the dew point of sulfur; accordingly, the catalyst is loaded with elemental sulfur and must periodically be regenerated.
Various proposed catalysts quickly lose their activity due to absorption of sulfur dioxide or sulfatization. In some of the processes, the catalyst remains sufficiently active for only 30 to 90 days. The proposed catalysts are bauxite, aluminosilicate zeolites, active carbon, active components in the form of the oxides or sulfides of tungsten, vanadium, chromium, iron, cobalt, nickel, copper, molybdenum, silver, and manganese on inactive porous carrier materials, as well as alkali metal sulfides and combinations of alkali metal oxides with alkaline earth metal oxides.
Thus, so far, no satisfactory process is available that permits the selective catalytic oxidation of hydrogen sulfide into elemental sulfur taking place entirely in the gas phase to be performed within the thermodynamically favorable temperature range, particularly the low temperature range, at a high reaction rate, particularly with a high degree of conversion, in long-term operation or continuous operation. For the solution of this problem, there is now proposed an economical process which does not show the disadvantages indicated hereinbefore.